The present invention relates to a blend of a tapered styrene-butadiene triblock thermoplastic elastomer with a rigid thermoplastic polymer. More particularly the present invention relates to such a blend utilizing low molecular weight tapered triblock elastomers having improved mechanical properties and ease of preparation.
It is previously known in the art to prepare blends of thermoplastic elastomers and in particular tapered styrene-butadiene block copolymer elastomers. In U.S. Pat. Nos. 3,906,057 and 3,906,058 there are disclosed blends of polystyrene or impact modified polystyrene polymers with tapered styrene-butadiene triblock elastomers. However, the triblock elastomers had high molecular weights in excess of 150,000 and preferably between 200,000 and 500,000. Such high molecular weight triblock elastomers are difficult to incorporate into thermoplastic resins due to the fact that the same have extremely low or even zero melt flow rates. Precise blending conditions had to be observed (see U.S. Pat. No. 3,906,507, Col. 6, lines 25-35) to achieve the desired effect. The use of such high molecular weight triblock elastomers under more normal blending conditions requires addition of an extender oil such as a naphthalenic or paraffinic oil to produce a block copolymer composition which is flowable and easily incorporated into the thermoplastic polymer. Disadvantageously such extender oils detrimentally affect the physical properties of the resulting thermoplastic blend.